The present invention relates in general to integrated circuits, and in particular to a robust oscillator circuit that is insensitive to variations in temperature and power supply voltage level.
In various types of telecommunication systems a reference frequency is commonly used as an acquisition aid in the receiver module. In those applications, such as ATM and SONET, that require reference frequencies of 35 Mhz or higher, generating a stable reference signal at such high references places a high cost and size burden on the sub-system or module. Crystal oscillators or tank circuits that are commonly used to generate the reference frequency take up additional board space and can be expensive. Thus, a receiver that does not require an externally generated reference frequency is very desirable. Internally generated reference frequencies, however, tend to be less accurate than their external counterparts. This is because on-chip oscillator circuitry is more susceptible to variations in temperature and power supply voltage. There have been various types of oscillator circuits developed that offer some degree of temperature and power supply independence. However, most require complex circuitry often relying on the temperature behavior of bipolar transistors. These types of circuits thus require an optimized bipolar process or a combination of bipolar and complementary metal-oxide-semiconductor (CMOS) process.
There is a need for a cost-effective oscillator circuit that generates a well controlled and stable reference frequency that is insensitive to temperature and power supply variations.